1. Field of the Invention
The present invention relates to a solid-state imaging device, which can change resolution, and to a method of driving the solid-state imaging device.
2. Description of the Related Art
A solid-state imaging device is used for the following image sensors. One is a one-dimensional image sensor used for reading an image in a scanner. Another is a two-dimensional image sensor used for reading a two-dimensional image such as a digital camera and a mobile phone with a camera. For example, the following solid-state imaging device has been disclosed in U.S. Pat. Appln. Publication No. US 2005/0069293 A1. The solid-state imaging device reads an image after adding a signal charge generated in a photosensitive pixel to change resolution. The solid-state imaging device disclosed in the foregoing Publication is provided with a plurality of charge transfers between photosensitive pixels and charge detectors. The charge transfer temporarily stores a signal charge generated in each photosensitive pixel, and transfers the stored signal charge to the charge detector when a pulse signal is applied. The charge detectors detect the signal charge, and then, convert it to an electric signal. A plurality of transistors is interposed between the charge transfers and output buffer.
The foregoing transistors are selectively turned on in accordance with the resolution of a read signal to be output. For example, in high resolution, the transistors are successively selected and turn on, and then, electric signals converted by the charge detectors are sent to the output buffer. In intermediate resolution, some of the transistors are simultaneously selected and turn on, and then, electric signals converted by some of the charge detectors are added, and thereafter, sent to the output buffer. In low resolution, all of the transistors are simultaneously selected and turn on, and then, all electric signals converted by the charge detectors are added, and thereafter, sent to the output buffer. As described above, according to the solid-state imaging device disclosed in the foregoing U.S. Publication, the transistors are selectively turned on. In this way, the resolution is readily changed.
However, the signal charge is transferred in a vertical direction with respect to the array direction of photosensitive pixels. For this reason, an array pitch of the photosensitive pixel becomes narrow, and thereby, the width of each transfer electrode of the charge transfer is narrowed. This is a factor of reducing a charge storable in each transfer electrode. The storage charge of each transfer electrode is reduced, and thereby, this is a factor of reducing the sensitivity. In order to solve the foregoing disadvantage, the electrode length of each transfer electrode is set longer to secure sufficient storage charge. By doing so, time is taken to transfer a charge, and this is a factor of transfer failure of the charge.